1. Field of the Invention
This invention relates to a method of sealing open ends of channels of a ceramic honeycomb structural body which is used for purifying harmful gases such as automobile exhaust gas or industrial-plant exhaust gas, and more particularly the invention relates to a method of sealing open ends of channels of a ceramic honeycomb structural body which forms a heat exchanger or a ceramic filter for removing particulates from automobile exhaust gas or a ceramic honeycomb structural body for supporting catalysts.
2. Description of the Prior Art
Recently, ceramic honeycomb structural bodies have been used as catalyst carriers for purifying automobile exhaust gas or as heat exchangers, and much attention has been paid to various advantages of such ceramic honeycomb structural bodies; namely, that pressure loss of gas flow therethrough is low because a large number of substantially straight and parallel channels are uniformly distributed in the ceramic honeycomb structural body. Additionally, the surface area per unit volume is large, and the ceramic honeycomb structural body can be heated up to a high temperature by using only a small amount of heat energy because the channels thereof are surrounded by thin walls.
For instance, it is known to make a ceramic honeycomb filter by taking advantage of the fact that the ceramic honeycomb structural body has a large surface area per unit volume and that the partition walls or inside walls of the ceramic honeycomb structural body defining a large number of channels therein are porous and very thin. More specifically, certain channels of the ceramic honeycomb structural body are sealed by suitable sealing material at one end thereof, while the remaining unsealed channels of the ceramic honeycomb structural body are subsequently sealed by a suitable sealing material at the opposite ends thereof. Hence, the partition walls form filtering layers for providing a large filtering area per unit volume of the ceramic filter while ensuring a low pressure therethrough. In such a filter, the thin porous partition walls capture particulates from the exhaust gas passing therethrough, thereby acting as an effective filtration system. Accordingly, the sealing material which seals the end openings of the channels of such ceramic honeycomb filter is required to be tightly bonded to the peripheral walls and partition walls so as to perfect sealing without any leakage of the dust-containing gas. Thus, reliable sealing of the open ends of the ceramic honeycomb structural body channels is a very important point in producing the ceramic honeycomb filter. When the ceramic honeycomb structural body is used as a carrier of catalysts, open ends of the channels in the proximity of the outer peripheral wall of the honeycomb structural body are sometimes sealed by a suitable sealing material for the purpose of improving the mechanical strength thereof, and the sealing material of such catalyst-supporting ceramic honeycomb structural body is also required to be tightly bonded to the partition walls of the honeycomb structural body.
As to the sealing material for sealing open ends of channels of a ceramic honeycomb structural body for the purpose of producing a ceramic honeycomb filter, U.S. Pat. No. 4,297,140 discloses manganese-cordierite system sealing material which is melted and foamed in the process of firing so as to seal the channels of the ceramic honeycomb structural body. Japanese patent application Laid-open Specification No. 42316/82 discloses the sealing of channels of a ceramic honeycomb structural body by fitting and bonding of suitable sealing material in the channels substantially without liquid-phase reaction, wherein the difference of the coefficients of thermal expansion between the honeycomb structural body and the sealant must be within a certain limited range and the sealing material is bonded substantially without liquid-phase reaction during firing. Furthermore, Japanese patent application Laid-open Specification No. 42317/82 discloses using alumina cement as a sealing material, whose heat-resistance is improved by using a specified concentration range of calcium oxide (CaO).
It should be noted that sealing by melting and foaming of a manganese-cordierite system sealing material during firing can provide tight sealing of the channels of the honeycomb structural body because of the comparatively low thermal expansion of the sealing material and the bonding, under vitrified molten condition, of the sealing material. However, the presence of manganese in the sealing material as a fusing-aid agent results in a very low melting point of about 1,200.degree. C., such that the sealing material has an insufficient heat-resistance for use as a sealant in a ceramic honeycomb filter which will be exposed to high temperature carbonous particulates captured by the honeycomb filter. If the content of the fusing-aid agent is limited to improve the heat-resistance, the increased melting and vitrification temperature of 1,300.degree. C., or higher, results in diffusion of the sealing material into the partition walls of the channels at the time of sealing the channels of a cordierite body.
When the sealing material is placed in, and bonded to the channels of the honeycomb structural body, substantially without liquid-phase reaction, (although a ceramic honeycomb filter with a good thermal shock resistance can be produced because of the small difference in thermal expansion between the sealant and the honeycomb structural body) there is a shortcoming in that the tightness of the sealing of channels is insufficient because of the firing shrinkage of the sealing material during solid phase reactions thereof. Hence, the firing tends to cause gaps between the sealant and the partition walls of the honeycomb structural body channels.
Many studies have been carried out on low-thermal-expansion cordierite compositions (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2), and as a result, a low thermal expansion cordierite composition with a coefficient of thermal expansion of not more than 11.0.times.10.sup.-7 /.degree.C., between 25.degree.-1,000.degree. C., has been obtained near the the theoretical cordierite composition (51.3% of SiO.sub.2, 34.9% of Al.sub.2 O.sub.3, 13.8% of MgO); more particularly, in the range of chemical composition consisting of 43-52% of SiO.sub.2, 33-39% of Al.sub.2 O.sub.3 and 12-18% of MgO. Such low thermal expansion cordierite is produced by carefully reducing impurities in the raw material batch, properly utilizing the orientation of the kaolin material, and prudently checking the raw materials such as the magnesia source. On the other hand, as to the reduction of firing shrinkage, which is necessary to effectuate the sealing of the channels in the honeycomb body, addition of other fusing agents or foaming agents is known. However, until now, there has not been any cordierite material batch composition suitable for sealing the open ends of channels in a low thermal expansion ceramic honeycomb structural body, wherein the material has a low intrinsic firing shrinkage and a finite value of volumetric expansion.